The present invention relates to a lift locking system for an elevatable horizontal platform and, more particularly, to a system for automatically locking the platform of a chain driven sheet stacker against free fall movement in the event of a chain failure.
Chain driven stackers, such as those used to accumulate a vertical stack of corrugated paperboard sheets from a conveyed serial line of sheets and to discharge the sheet stack for further processing, are well known in the art. One such stacker is shown in U.S. Pat. No. 3,905,595. In a downstacker, sheets are fed onto a stacker platform initially positioned at the top of a supporting frame and the platform is continuously lowered as the stack of sheets builds.
In a somewhat similar device, commonly called an upstacker, sheets are fed by an infeed conveyor onto a vertically fixed platform and the infeed conveyor is raised vertically to follow the rising stack of sheets.
Similar hydraulically operated lift chain devices may be used with either a downstacker or an upstacker and one common lift chain arrangement is shown in U.S. Pat. No. 4,065,122, that configuration particularly adapted for an upstacker. In applying a similar lift chain arrangement to the platform of a downstacker, a pair of lift cylinders are mounted to the frame adjacent one edge of the platform, the rod end of the cylinder is attached to one end of the lift chain which extends vertically over an upper idler sprocket and downwardly to attachment at its other end to the lift platform. The edge of the lift platform opposite the lift chain attachments is supported by a pair of timing chains each secured at one end to the upper frame, extending vertically down over one idler sprocket at that edge of the platform, generally horizontally alongside the platform to and around a second idler sprocket adjacent the lift chain edge of the platform, and then vertically downwardly where it is secured at its other end to the lower frame. Because the lift chains and timing chains operate around toothed sprockets, the chains are maintained in synchronism and the lift platform is always maintained horizontal and level. The timing chains also help equalize the load on the platform and therefore the pressure in the lift cylinders in the situation when the stack of sheets is formed nearer one side of the lift platform.
In the event of failure of a lift chain or a timing chain, a corner of the platform will tend to free-fall and, if no other chain subsequently fails, may result in a canting and jamming of the lift platform. A potentially more serious situation could result from multiple chain failure and substantial free-fall of the entire lift platform. Therefore, various devices to rapidly lock the platform and prevent significant free-fall in the event of failure of a lift chain or cable have been devised.
Above identified U.S. Pat. No. 4,065,122 discloses a locking device attached directly to the end of the lift chain which is activated to bias rotatable locking jaws into contact with the lift track as a result of loss of tension in the lift chain because of failure. Similarly, U.S. Pat. No. 4,725,185 shows lift platform for the rear of a truck with similar rotatable locking jaws activated by loss of tension in the lift cable to engage the lift track and halt free-fall descent of the platform.
Because each of the foregoing locking devices is subject, to some extent at least, to frictional engagement between the engaging locking members and, consequently, potentially substantial component wear, the amount of free-fall descent of the platform after lift chain or cable failure may be substantially uncertain or even dangerously excessive.
It would be desirable, therefore, to have a lift locking system for a downstacker or the like which would be operable immediately upon lift or timing chain failure to positively lock the lift platform against free-fall descent, not only at the corner where chain failure occurs, but at all four corners of the platform. The locked system would allow convenient access to the downstacker to effect necessary repairs. It would also be advantageous to have such a system which is selectively actuable on a fully operative downstacker to lock the lift platform in any desired position, as for inspection or maintenance purposes.